Document --package-format=<fmt> option. I had implemented that option some
time ago, but decided to not expose it in the man-page, since portmaster
will not necessarily install the latest version if installing from local
Jpackages is requested and there exist multiple package files that only
differ in archive format. But since a non-standard package format allows
to choose between an optimization for speed or for disk space, this is a
useful option that shall be described in the man page.

Approved by: antoine (implicit)